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### When to Use Q

Posted: Tue Jan 21, 2020 7:59 pm
5I.25.)
A reaction mixture is prepared by mixing 0.100 mol SO2, 0.200 mol NO2, 0.100 mol NO, and 0.150 mol SO3 in a reaction vessel of volume 5.00 L. The reaction SO2(g) 1 NO2(g) ∆ NO(g) 1 SO3(g) is allowed to reach equilibrium at 460 8C, when Kc 5 85.0. What is the equilibrium concentration of each substance?

Why do we need to calculate Q to determine the direction of the reaction, but we didn't for the problem in lecture, where we had a solution with 0.100 M nitrous acid and 0.150 potassium nitrate? (Week 2 Friday)

### Re: When to Use Q

Posted: Tue Jan 21, 2020 8:38 pm
In the case of the lecture problem, I think it was assumed that the reaction would just proceed forward. However, unless you're working with acid-base equilibria related reactions, I think it's always safest to calculate Q to make sure that the direction of the reaction is correct, even if the problem doesn't explicitly state to.

### Re: When to Use Q

Posted: Tue Jan 21, 2020 8:39 pm
wait so for this problem we had to make sure which direction it went in?

### Re: When to Use Q

Posted: Tue Jan 21, 2020 8:40 pm
The quotient, Q, shows us the concentration of the reactants and products at any time while K shows us the unique concentrations for each when at equilibrium. Because reactions are in constant motion to reach equilibrium, you can use Q and compare to K to determine if the reaction will move forward or reverse.